Superheated phase changing nanodroplets for hydrocarbon reservoir applications

ABSTRACT

A method includes injecting an injection fluid through a well and to a depth of a formation, where the injection fluid includes phase-changing nanodroplets having a liquid core and a shell. The method also includes exposing the phase-changing nanodroplets to an external stimulus at the depth of the formation, wherein the liquid core of the phase-changing nanodroplets undergoes a liquid-to-vapor phase change causing the phase-changing nanodroplets to expand, and stimulating the formation at a near wellbore region by expansion of the phase-changing nanodroplets.

BACKGROUND

Hydrocarbon resources, including oil and gas, are typically locatedbelow the surface of the earth in subterranean porous rock formations.To access these resources, wells are drilled to extract the hydrocarbonfluids from the reservoir. However, drilling may also damage theformation physically or chemically due to the interaction between thedrill bit, a reamer, or the drilling fluid and the rocks and minerals inthe formation. For example, mud filtrate within the drilling mud maydeposit on the face of the wellbore, forming a layer (termed “filtercake” or “mud cake”) adhered to the wellbore wall. Additionally, thepores of the subterranean formation near the wellbore may be impacted bypore plugging (that is, formation damage) resulting from fines orfiltrate invasion either through transport phenomenon or by beingmechanically forced into the pores. Such damage mainly occurs at theinterface between the wellbore and the reservoir, called the “nearwellbore region” (NWR), or it may extend deeper within.

Hydrocarbon production traditionally may be stimulated by removing theformation damage elements to the wellbore walls and NWR by applying achemical (for example, acids) or by utilizing a mechanical solution.Such remediation may allow hydrocarbons from the reservoir to traversethrough the NWR and into the wellbore.

SUMMARY

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

In one aspect, embodiments disclosed herein relate to a method thatincludes providing an injection fluid with phase-changing nanodropletsand an aqueous-based fluid. The phase-changing nanodroplets include aliquid core and a shell. The injection fluid may then be injected into awellbore at a first temperature. After injection into the wellbore, thephase-changing nanodroplets are exposed to a second temperature that isgreater than or equal to a boiling point of the liquid core. Theexposure of the phase-changing nanodroplets to the second temperaturechanges a liquid in the liquid core to a vapor phase and expands thephase-changing nanodroplets to remove debris from the wellbore andsurrounding area by expansion of the phase-changing nanodroplets.

In another aspect, embodiments disclosed herein relate to a method thatincludes providing an injection fluid with phase-changing nanodropletsand an aqueous-based fluid. The phase-changing nanodroplets include aliquid core and a shell. The method may further include injecting theinjection fluid through a well and to a depth of a formation andexposing the phase-changing nanodroplets to an external stimulus. Uponexposure to the external stimulus, the liquid core of the phase-changingnanodroplets undergoes a liquid-to-vapor phase change causing thephase-changing nanodroplets to expand. The method may further includestimulating the formation at a near wellbore region using the expansionof the phase-changing nanodroplets.

In yet another aspect, embodiments disclosed herein relate to acomposition including 0.1 to 10 wt % phase-changing nanodroplets havinga liquid perfluorocarbon core and a shell encapsulating the liquidperfluorocarbon core, and 50 to 97 wt % of an aqueous-based injectionfluid selected from the group consisting of wellbore clean-up fluid,formation stimulation fluid, and combinations thereof.

Other aspects and advantages of the claimed subject matter will beapparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a well system for treating the wellbore and near wellboreregion of a reservoir according to one or more embodiments.

DETAILED DESCRIPTION

A major concern with using strong acids near the reservoir is that mudcake and other wellbore damage removal may be non-uniform due todifferences in the composition or thickness, leaving portions of thewellbore wall and NWR with debris and fluid blockages. Another issue isthat strong acids tend to instantaneously react with subterraneanformation material upon contact due to the amount of acid-reactivematerial present and the acid strength. This results in almost immediatedepletion of the strong acid, resulting in the creation of wide yetshallow (that is, not penetrating deep into the subterranean formationrelative to the distance from the wellbore) and non-uniform wormholesthrough the subterranean formation. These shallow-depth wormholes arenot as desirable as deep fluid pathways through the reservoir that wouldfunction as conduits for hydrocarbon fluid flow into the wellbore. Deepfluid pathways may enhance the rate and efficiency of hydrocarbonextraction from the reservoir.

Embodiments disclosed herein generally relate to compositions andmethods for wellbore cleanup and/or formation stimulation usingsuperheated phase-changing nanodroplets. The superheated phase-changingnanodroplets may include a fluid perfluorocarbon (PFC) compound coreencapsulated by a shell. One or more embodiments relate to superheatedphase-changing nanodroplets that may be included in an aqueous-basedinjection fluid. The liquid PFC core of superheated phase-changingnanodroplets may have a boiling point equal to a downhole temperaturecorresponding to a target depth downhole. Methods in accordance withsome embodiments may involve injecting an aqueous-based injection fluidcomprising phase-changing nanodroplets into a wellbore and exposing theinjection fluid to an elevated temperature downhole, resulting insuperheating the phase-changing nanodroplets and explosive vaporizationof the liquid PFC core of the nanodroplets. Vaporization of the liquidPFC core may cause rapid expansion of the phase-changing nanodroplets,generating an acoustic energy wave that may displace, break, or loosendebris such as mud cakes and fines from the walls of the wellbore andnear wellbore region (NWR).

In one or more embodiments, explosive boiling of the PFC core insuperheated phase-changing nanodroplets may occur, in part, due to thesurface tension provided by the surrounding shell. The core-shellstructure of the phase-changing nanodroplets may provide a pressuredifference, known as Laplace pressure, between the inside and outside ofthe shell of superheated phase-changing nanodroplets of the presentdisclosure. Laplace pressure may be described according to Equation 1,below.

$\begin{matrix}{{\Delta P} = {{P_{in} - P_{out}} = {\frac{2}{R}\gamma}}} & {{Eq}.1}\end{matrix}$

In Equation 1, R is the radius and y is the interfacial tension of agiven superheated phase-changing nanodroplet. Accordingly, it can beappreciated that the Laplace pressure may become greater in smallernanodroplets and at higher interfacial tensions. As derived fromEquation 1, the pressure inside disclosed superheated phase-changingnanodroplets may be calculated according to Equation 2, below.

$\begin{matrix}{P_{in} = {P_{out} + {\frac{2}{R}\gamma}}} & {{Eq}.2}\end{matrix}$

In one or more embodiments, the temperature at which a phase-changingnanodroplet may vaporize at a known pressure may be estimated accordingto the Antoine equation, shown below.

$\begin{matrix}{T = {\frac{B}{A - {\log_{10}p}} - C}} & {{Eq}.3}\end{matrix}$

In Equation 3, A, B, and C are component-specific constants that may beobtained empirically. As shown in Equation 3, a liquid at high pressuremay have a boiling point that is higher than the same liquid in ambientconditions. The pressure inside the phase-changing nanodroplets of oneor more embodiments may be elevated, and thus, the liquid PFC cores mayhave boiling points higher than that of the same PFC compounds in bulkat ambient pressure. Accordingly, the pressure differential may allowthe core to remain in liquid state (where in a free environment, thecore may otherwise be in a gas state), and the liquid core may remainstable until exposed to an external stimulus such as an increase intemperature or ultrasonic excitation. For example, one or moreultrasonic devices (e.g., downhole ultrasonic devices) may be used toemit ultrasonic waves to the phase-changing nanodroplets to initiate aliquid-to-gas phase change in the liquid core and expand thephase-changing nanodroplets.

In one or more embodiments, explosive vaporization of the PFC core maygenerate gas bubbles. “Cavitation” is the generation and energeticfailure of gas or vapor-filled voids within a liquid. Small vapor-filledbubbles may be formed when the pressure of a liquid is less than thevapor pressure of the liquid. These bubbles, once formed, may collapsewhen subjected to a greater pressure. The collapse of bubbles occurswhen the pressure exceeds the vapor pressure of the bubble. An acousticwave may provide this temporary pressure increase to induce bubblecollapse. Accordingly, some embodiment methods may include providingacoustic energy downhole to promote cavitation of gas bubbles formedfrom the rapid expansion of superheated phase-changing nanodropletsdisclosed herein.

The collapse of such bubbles may cause a high energy shock wave thatfurther forms a fluidic microjet. The resultant microjets from thecavitation of bubbles may have a speed of about 100 meters per second(m/s). Such speed in micro-sized fluid jets may lead to impact pressureson nearby solid surfaces of up to about 50 megapascals (MPa). Thefluidic microjets may be utilized in one or more embodiments to dislodgeor destroy debris in the wellbore and NWR.

In one or more embodiments, compositions and methods disclosed hereinmay be used for wellbore treatment and/or formation stimulation. Whilewellbore treatment and formation stimulation may include increasingpathways between a reservoir and a wellbore, it may be desirable toretain mud cake or other wellbore fluid residue on a portion of thereservoir face to restrict fluid access between the wellbore and variousformations in the subsurface. As described above, the liquid PFC core ofphase-changing nanodroplets may have a specific boiling point. Theboiling point may be equal to that of the temperature at a specificdownhole region. As such, explosive boiling of the phase-changingnanodroplets and treatment of the wellbore or NWR, may only occur at thetarget temperature. Accordingly, methods of using phase-changingnanodroplets having pre-selected boiling points for wellbore treatmentand/or formation stimulation may effectively remediate drilling damageat specific regions of the wellbore and NWR having temperatures greaterthan or equal to the pre-selected boiling points.

For example, as described above, encapsulating a liquid PFC core in ashell may increase the boiling point of the liquid PFC core. Further,the size of the liquid core encapsulated by a shell affects the boilingpoint of the liquid core, where generally, decreasing the size of theliquid core may increase the boiling point of the liquid core.Accordingly, a phase-changing nanodroplet may be tailor-made to have aliquid core with a selected boiling point by selection of the liquidcore and shell material and size. For example, a liquid used to form aliquid core may have an initial boiling point under an ambient pressure,when the liquid is not within the shell. When an amount of the liquid isencapsulated in a shell (forming a liquid core having a certain size),the boiling point of the liquid core within the shell is greater thanthe initial boiling point of the liquid. In such manner, the boilingpoint of a liquid core in a phase-changing nanodroplet may be designedto correspond with elevated temperatures of a targeted downhole region,which may allow for a delayed activation of the phase-changingnanodroplets until the nanodroplets reach the targeted downhole region.

FIG. 1 depicts a well system for treating the wellbore walls or NWR of areservoir according to one or more embodiments. The well system 1000depicts a geological formation 100 with an associated treatment system300 mounted on the surface 102 of the geological formation 100, wherethe surface 102 represents the surface of the earth. Surface 102 may belocated above water, under water, or under ice. Below the surface 102 isthe subsurface 104, which may include a reservoir 106. Reservoir 106 isa hydrocarbon-bearing formation.

Traversing the subsurface 104 is a wellbore 110. Wellbore 110 is definedby wellbore wall 112. The wellbore 110 traverses through the reservoir106 such that the wellbore 110 is in fluid communication with the NWR115 portion of the reservoir 106 at reservoir face 114.

With the associated treatment system 300, a derrick 302 is located onthe surface 102 to support a work string 320 positioned in the wellbore110. The work string 320 comprises a pipe 322 that runs from the surface102 downhole in the wellbore 110 that terminates near the reservoir 106with a coupled downhole tool 330. The work string 320 and the wellborewall 112 defines a wellbore annulus 116 along the length of the wellbore110. Wellbore annulus 116 is the void in the wellbore 110 not occupiedby the work string 320.

On the surface 102, the treatment system 300 includes surface emulsiongeneration tool 310, which is configured to generate an emulsion ofphase-changing nanodroplets in aqueous-based injection fluid utilizedfor treatment of the wellbore wall 112 and/or the NWR 115. Surfaceemulsion generation tool 310 is fluidly coupled to the downhole tool 330using liquid treatment conduit 312, which is fluidly coupled to thedownhole tool 330 via pipe 322. The phase-changing nanodroplets 340 areintroduced into the wellbore 110 via liquid treatment discharge 332 ofdownhole tool 330.

Downhole tool 330 may be used to inject phase-changing nanodroplets 340into the wellbore 110 at any depth from the surface 102. Thephase-changing nanodroplets 340 may be injected at a depth having atemperature below the boiling point of the PFC core of phase-changingnanodroplets 340. As such, the phase-changing nanodroplets may naturallytravel downhole, and upon reaching a downhole temperature similar tothat of the PFC core boiling point, the phase-changing nanodroplets mayexpand and treat the wellbore or NWR.

Treatment system 300 also includes a surface acoustic signal generator316, which may be used to generate an acoustic or ultrasonic signal foruse in treatment of the wellbore wall 112 and/or the NWR 115 of thereservoir 106. Surface acoustic signal generator 316 is signally coupledto the downhole tool 330 using an acoustic signal conduit 318, which mayrun along the interior of pipe 322. Transmission of acoustic orultrasonic signals into wellbore 110 and the NWR 115 originates from anacoustic signal transmitter 334 of downhole tool 330.

While the well system 1000 shown in FIG. 1 shows an example using aderrick 302 system to hold a work string 320 in the well, other toolsand system configurations known in the art may be used to provide afluid conduit downhole, through which an emulsion of phase-changingnanodroplets may be sent downhole from a surface emulsion generationtool 310. For example, in some embodiments, a phase-changing nanodroplettreatment system may be incorporated into a drilling operation, where anemulsion of phase-changing nanodroplets may be sent downhole via a drillstring. In some embodiments, a phase-changing nanodroplet treatmentsystem may be incorporated into a production operation (where a well hasbeen completed and production from the well has already been initiated).For example, a phase-changing nanodroplet treatment system may beincorporated into a production operation showing a decline in theproductivity index (PI), where an emulsion of phase-changingnanodroplets may be sent downhole via a fluid conduit to treat a portionof the well and increase PI. Further, other tools and systems known inthe art may be used to transmit signals, e.g., acoustic, ultrasonic, orelectric signals, downhole.

Composition of Phase-Changing Nanodroplets

As previously described, the present disclosure relates to the use ofphase-changing nanodroplets for wellbore cleanup and/or formationstimulation. Phase-changing nanodroplets in accordance with the presentdisclosure include a liquid PFC core encapsulated by a shell.

Various PFC compounds may be included in the liquid core of thephase-changing nanodroplets. Specific PFC compounds may be selected foruse in phase-changing nanodroplets of one or more embodiments accordingto the downhole temperature of the target treatment region. Suitable PFCcompounds may have a boiling point when encapsulated in a shell similarto that of the temperature of the target region of the wellbore or NWR(e.g., within ±5° C. of the downhole temperature of the target region).Exemplary PFC compounds and their boiling points includeoctafluoropropane (−39° C.), perfluorobutane (−2° C.), perfluoropentane(30° C.), perfluorohexane (59° C.), perfluorohexyl bromide (97° C.),perfluorooctyl bromide (142° C.), and perfluoro-15-crown-5-ether (146°C.). As previously described, a PFC compound in nanodroplet form mayhave a higher boiling point than the same PFC compound at ambientconditions. For example, an octafluoropropane liquid core may have anincreased boiling point when encapsulated in a nanodroplet shell, e.g.,a boiling point of greater than 37° C. in a nanodroplet having adiameter of about 700 microns. In some embodiments, a mixture of atleast two PFC compounds may be included in the liquid core ofphase-changing nanodroplets. The boiling point of the PFC core may rangefrom about 50 to 200° C. For example, PFC cores may have a boiling pointranging from a lower limit of any of 50, 60, 70, 80, 90, 100, 110, and120° C. to an upper limit of any of 130, 140, 150, 160, 170, 180, 190,and 200° C., where any lower limit may be paired with any mathematicallycompatible upper limit.

In one or more embodiments, the PFC core may be included in aphase-changing nanodroplet in an amount ranging from 50 to 80%, byvolume, based on the total volume of the nanodroplet. For example,phase-changing nanodroplets according to the present disclosure mayinclude a PFC in an amount having a lower limit of any of 50, 55, 60,65, and 70% and an upper limit of any of 60, 65, 70, 75, and 80% whereany lower limit may be paired with any mathematically compatible upperlimit.

The liquid PFC core may be encapsulated by a shell made of variousmaterials. Suitable shell materials include, but are not limited to,polymers, surfactants, and lipids. For example, the shell of aphase-changing nanodroplet may be made of lipids such as phospholipidsand bovine serum albumin (BSA); surfactants including perfluorosulfonicacids such as perfluorooctanesulfonic acid (PFOS) andperfluorobutanesulfonic acid (PFBS), perfluorocarboxylic acids such asperfluorooctanoic acid (PFOA) and perfluorohexanoic acid (PFHxA), andother fluorosurfactants such as nonionic ethoxylated fluorosurfactantsincluding Zonyl FSO from Sigma Aldrich; or polymers such aspoly(lactic-co-glycolic acid), zwitterionic polymers, fluoroalkylpolymers including fluoroalkyl methacrylate, fluoroalkyl2-trifluoromethylacrylate, fluoroalkyl(C6) ethyl acrylate, fluoroalkylpentacarbonylmanganese(I), co-polymers of poly(ethylene) glycol andpoly(l-lactic) acid (PEG-PLLA), and co-polymers of PEG andpolycaprolactone (PEG-PCL), among others.

Phase-changing nanodroplets according to the present disclosure mayinclude a shell in an amount ranging from 20 to 50%, by volume, based onthe total volume of the nanodroplet. For example, in one or moreembodiments, the shell may be included in the phase-changing nanodropletin an amount having a lower limit of any of 20, 25, 30, 35, and 40% andan upper limit of any of 30, 35, 40, 45, and 50%, where any lower limitmay be paired with any mathematically compatible upper limit. In someembodiments, the core material may form a first volume of thephase-changing nanodroplet, and the remaining volume of a phase-changingnanodroplet may be the shell material. For example, a phase-changingnanodroplet may have a PFC core forming 50 to 80% of the volume of thedroplet and a shell forming the remaining 50 to 20% of the volume of thedroplet.

In one or more embodiments, phase-changing nanodroplets have a sizeranging from 10 to 1000 nm in average diameter. For example, thephase-changing nanodroplets may range in size from a lower limit of anyof 10, 20, 50, 100, 150, 200, 250, 300, and 400 nm to an upper limit ofany of 500, 600, 700, 750, 800, 850, 900, 950, and 1000 nm, where anylower limit may be paired with any mathematically compatible upperlimit.

Phase-changing nanodroplets according to the present disclosure may beprepared by providing a PFC-in-water emulsion stabilized by shellmaterial. Such emulsions may be generated by sonication, amalgamation,homogenization, or microfluidics.

For example, methods of one or more embodiments may include dissolvingthe shell material into an organic solvent to provide an organicsolution. Suitable organic solvents may be immiscible with water.Examples of suitable organic solvents include chloroform anddichloromethane. The organic solution may then be added to an aqueousphase. The aqueous phase includes water. Then, the solution may beagitated at a suitable temperature for an amount of time to allow anemulsion including a continuous phase and nanodroplets to form. Theemulsion may be agitated at a temperature ranging from 0 to 50° C. Inone or more embodiments, the emulsion may be agitated for an amount oftime ranging from 2 to 24 hours.

After being agitated at a suitable temperature for an amount of time,the nanodroplets may be separated from the continuous phase and washedto provide pure phase-changing nanodroplets according to the presentdisclosure.

Other examples of methods that may be used to form phase-changingnanodroplets include methods that have been used to form nanodropletsfor bio-applications. For example, suitable methods for formingphase-changing nanodroplets are described in Santiesteban et al. (2019)(encapsulating perfluoropentane in phospholipid shells), Wilson et al.(2011) (synthesizing perfluoropentane nanodroplets surrounded by a shellof bovine serum albumin (BSA)), Hallam et al, (2018) (synthesizingnanodroplets of perfluorohexane stabilized by a fluorosurfactant), andPisani et al. (2006) (describing a method to produce nanodroplets ofperfluoropentane, perfluorohexane, perfluorooctane, and perfluorodecalinstabilized by polymeric shells), each of which are incorporated hereinby reference.

Composition of Injection Fluid Including Phase-Changing Nanodroplets

As described above, the phase-changing nanodroplets in accordance withthe present disclosure may be added to aqueous-based injection fluid.The aqueous-based injection fluid includes water. The water may bedistilled water, deionized water, tap water, fresh water from surface orsubsurface sources, production water, formation water, natural andsynthetic brines, brackish water, natural and synthetic sea water, blackwater, brown water, gray water, blue water, potable water, non-potablewater, other waters, and combinations thereof, that are suitable for usein a wellbore environment. Contaminants in the water such as salts,ions, minerals, organics, and combinations thereof may reduce theinterfacial tension between the droplets and the continuous medium,which would reduce the inner pressure of the droplets and thus boilingtemperature of the phase-changing nanodroplets. Thus, according toembodiments of the present disclosure, water without such contaminantsmay be used, or such contaminants may be measured and used incalculations (e.g., see Eq. 1) for designing a selected boilingtemperature of the phase-changing nanodroplets.

In one or more embodiments, the injection fluid may contain water in arange of from about 50 wt % to 97 wt % based on the total weight of theinjection fluid. In one or more embodiments, the injection fluid maycomprise greater than 70 wt % water based on the total weight of theinjection fluid.

In one or more embodiments, the water used for the injection fluid mayhave an elevated level of salts or ions versus fresh water, such assalts or ions naturally-present in formation water, production water,seawater, and brines. In one or more embodiments, salts or ions areadded to the water used to increase the level of a salt or ion in thewater to effect certain properties, such as density of the injectionfluid or to mitigate the swelling of clays that come into contact withthe drilling fluid. Without being bound by any theory, increasing thesaturation of water by increasing the salt concentration or otherorganic compound concentration in the water may increase the density ofthe water, and thus, the injection fluid. Suitable salts may include,but are not limited to, alkali metal halides, such as chlorides,hydroxides, or carboxylates. In one or more embodiments, salts includedas part of the aqueous-based fluid may include salts that disassociateinto ions of sodium, calcium, cesium, zinc, aluminum, magnesium,potassium, strontium, silicon, lithium, chlorides, bromides, carbonates,iodides, chlorates, bromates, formates, nitrates, sulfates, phosphates,oxides, and fluorides, and combinations thereof. Without being bound byany theory, brines may be used to create osmotic balance between theinjection fluid and portions of the subterranean formation, such asswellable clays.

In one or more embodiments, the injection fluid may comprise one or moresalts in an amount that ranges from about 1 to about 300 ppb (pounds perbarrel). For example, the injection fluid may contain the one or moresalts in an amount ranging from a lower limit of any of 1, 10, 50, 80,100, 120, 150, 180, 200, 250 and 280 ppb, to an upper limit of any of20, 30, 40, 50, 70, 100, 120, 150, 180, 200, 220, 240, 260, 280 and 300ppb, where any lower limit can be used in combination with anymathematically-compatible upper limit.

In one or more embodiments, the injection fluid may have a pH close toneutral. For example, the injection fluid of one or more embodiments mayhave a pH ranging from 5 to 9, from 5 to 8, from 5 to 7, from 6 to 9,from 6 to 8, from 6 to 7, from 7 to 9, or from 7 to 8.

In one or more embodiments, the injection fluid may comprise a suitableamount of the previously described phase-changing nanodroplets. In oneor more embodiments, the phase-changing nanodroplets are present in theinjection fluid in an amount ranging from about 0.1 to 10 wt % (weightpercent). For example, the injection fluid may contain thephase-changing nanodroplets in an amount ranging from a lower limit ofany of 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 wt % toan upper limit of any of 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0,9.5, and 10 wt % where any lower limit may be used in combination withany mathematically compatible upper limit.

In some embodiments, the injection fluid may include additives such asviscosifiers and/or emulsifiers to stabilize the phase-changingnanodroplets. Examples of viscosifiers include, but are not limited to,xanthan gum and polymers used in enhanced oil recovery such assulphonated polyacrylamides, e.g., AN-132 (copolymer of acrylamide (AM)with acrylamide tertiary-butyl sulfonic acid (ATBS or AMPS) with 32 mol% degree of sulfonation) and AN-125 (copolymer of AM/AMPS 75/25 mol %(25 mol % degree of sulfonation)). Viscosifiers may be included in theinjection fluid in a concentration at or below 0.02 wt %. For example,viscosifiers may be included in a concentration at or below, 0.02 wt %,at or below 0.015 wt %, at or below 0.01 wt %, or at or below 0.005 wt%. Suitable emulsifiers include, for example, anionic surfactants andpolymers, cationic surfactants and polymers, and zwitterionic surfactantand polymers, among others. The concentration of emulsifier that may beincluded in the injection fluid may depend on the critical micelleconcentration of the emulsifier included. In general, the emulsifier maybe injected at concentration above the critical micelle concentration.

Embodiments of the present disclosure may provide at least one of thefollowing advantages. Superheated phase-changing nanodroplets accordingto embodiments of the present disclosure may be used for targetedtreatment of a wellbore and NWR. The PFC core of superheatedphase-changing nanodroplets may be formulated to have a boiling pointsimilar or equal to the downhole temperature of a target treatmentregion. As such, wellbore cleanup and/or formation stimulation may berestricted to a specific region of the wellbore having a downholetemperature range using the compositions disclosed herein havingcorresponding boiling points. Similarly, phase-changing nanodroplets maybe formulated to expand deep into the formation, resulting in moreuniform, targeted stimulation treatments.

Additionally, by using injection fluid and phase-changing nanodropletmixtures according to embodiments of the present disclosure, aninjection fluid having a neutral or close to neutral pH may be used as asubstitute for acid stimulation.

EXAMPLES

Poly(lactic-co-glycolic acid) (PLGA) was provided byBoehringer-Ingelheim, perfluoropentane, perfluorohexane,perfluorooctane, and perfluorodecalin were provided by Fluorochem,phospholipids we provided for Avanti Polar Lipids, Zonyl FSOfluorosurfactant and BSA were provided by Sigma Aldrich.

Examples of phase-changing nanodroplets include perfluoropentaneencapsulated in PLGA, perfluorohexane encapsulated in PLGA,perfluorooctane encapsulated in PLGA, perfluorodecalin encapsulated inPLGA, perfluoropentane encapsulated in a phospholipid shell;perfluorohexane nanodroplets stabilized with Zonyl FS 0fluorosurfactant, perfluoropentante encapsulated in bovine serum albumin(BSA).

Although only a few example embodiments have been described in detailabove, those skilled in the art will readily appreciate that manymodifications are possible in the example embodiments without materiallydeparting from this invention. Accordingly, all such modifications areintended to be included within the scope of this disclosure as definedin the following claims.

1.-10. (canceled)
 11. A method comprising: providing an injection fluidcomprising phase-changing nanodroplets and an aqueous-based fluidwherein the phase-changing nanodroplets comprise: a liquid core; and ashell; injecting the injection fluid through a well and to a depth of aformation; exposing the phase-changing nanodroplets to an externalstimulus at the depth of the formation, wherein the liquid core of thephase-changing nanodroplets undergoes a liquid-to-vapor phase changecausing the phase-changing nanodroplets to expand; and stimulating theformation at a near wellbore region by expansion of the phase-changingnanodroplets.
 12. The method of claim 11, wherein the external stimuluscomprises ultrasonic waves emitted from an ultrasonic device.
 13. Themethod of claim 11, wherein the external stimulus comprises a downholetemperature of the formation, where the downhole temperature is greaterthan or equal to a boiling point of the liquid core.
 14. The method ofclaim 11, wherein the liquid core is a perfluorocarbon compound selectedfrom the group consisting of octafluoropropane, perfluorobutane,perfluoropentane, perfluorohexane, perfluorohexyl bromide,perfluorooctyl bromide, perfluoro-15-crown-5-ether, and combinationsthereof.
 15. The method of claim 11, wherein the shell is a polymershell selected from the group consisting of poly(lactic-co-glycolic)acid (PLGA), fluoroalkyl methacrylate, fluoroalkyl2-trifluoromethylacrylate, fluoroalkyl(C6) ethyl acrylate, fluoroalkylpentacarbonylmanganese(I), co-polymers of poly(ethylene) glycol andpoly(l-lactic) acid (PEG/PLLA), co-polymers of poly(ethylene) glycol andpolycaprolactone (PEG/PCL), and combinations thereof.
 16. The method ofclaim 11, wherein the shell is a surfactant shell made from a nonionicethoxylated fluorosurfactant.
 17. A composition comprising: 0.1 to 10 wt% phase-changing nanodroplets comprising: a liquid perfluorocarbon core;and a shell encapsulating the liquid perfluorocarbon core; and 50 to 97wt % of an aqueous-based injection fluid selected from the groupconsisting of wellbore clean-up fluid, formation stimulation fluid, andcombinations thereof.
 18. The composition of claim 17, furthercomprising an additive selected from the group consisting of anemulsifier, a viscosifier, and a combination thereof.
 19. Thecomposition of claim 17, wherein the liquid perfluorocarbon core isselected from the group consisting of octafluoropropane,perfluorobutane, perfluoropentane, perfluorohexane, perfluorohexylbromide, perfluorooctyl bromide, perfluoro-15-crown-5-ether, andcombinations thereof.
 20. The composition of claim 17, wherein the shellis a polymer shell selected from the group consisting ofpoly(lactic-co-glycolic) acid (PLGA), fluoroalkyl methacrylate,fluoroalkyl 2-trifluoromethylacrylate, fluoroalkyl(C6) ethyl acrylate,fluoroalkyl pentacarbonylmanganese(I), co-polymers of poly(ethylene)glycol and poly(l-lactic) acid (PEG/PLLA), co-polymers of poly(ethylene)glycol and polycaprolactone (PEG/PCL), and combinations thereof.